A liquid crystal display (LCD), which is one example of image display apparatuses, is comprised of a LCD element; a backlight positioned under the LCD element, for feeding light to the LCD element; and a circuit board or the like for actuating the LCD element. In the LCD element, an electrode, alignment layer etc. formed from, for example, a transparent conductive thin film are laminated on two transparent glass substrates; the glass substrates are opposed with a specified spacing between such that their laminated surfaces face each other; a liquid crystal is confined between the two glass substrates; and a polarizing plate is disposed outside both glass substrates.
FIG. 10 shows a conventional backlight of the edge light type. The backlight 500 comprises an optical waveguide 503 constituted by a transparent synthetic resin plate which guides light incident on its end faces in a direction parallel with its main faces and allows the light to leave from one of the main faces; fluorescent tubes 501 each of which is a light source disposed in the vicinity of an end face 503a of the optical waveguide 503 so as to extend substantially in parallel with the end face 503a; reflecting covers 502 each of which covers the substantially entire length of a fluorescent tube 501; a diffusion sheet (not shown) positioned on the upper face (i.e., upper main face) of the optical waveguide 503, for diffusing light coming from the optical waveguide 503; and a reflecting sheet (not shown) positioned on the lower face (i.e., lower main face) of the optical waveguide 503, for upwardly reflecting light coming from the optical waveguide 503.
In the backlight 500 of such a structure, light beams emanated from the light source 501 are guided by the optical waveguide 503 away from the light source 501 with some light beams being reflected by the reflection sheet such that they go out of the optical waveguide 503 through its upper face. The outgoing light beams are diffused by the diffusion sheet to enter the LCD element (not shown). As a result, the whole LCD element is uniformly irradiated by the light beams.
Recently, there have been increasing demands for lightweight thin LCDs. To meet the demands, edge light type backlights are widely used in which a tubular light source such as cold cathode tubes is placed on the end faces of a LCD. In the field of LCDs for use in monitors and similar apparatuses, technology for producing high-fineness and high-brightness LCDs has been progressing. For achieving increased brightness with a view to coping with such a trend, it is necessary to increase the output of the cold cathode tubes or use more cold cathode tubes.
If more cold cathode tubes are used, the volume of the cold cathode tubes increases with the result that the region occupied by the light source becomes larger. If a cold cathode tube having increased output is employed, difficulties are encountered in producing thin LCDs because the size of a cold cathode tube increases as its output increases. Further, there are restrictions on electric power consumption, and therefore the output of the light source cannot be increased without careful consideration.
As liquid crystals used for LCDs, the Twisted Nematic (TN) Mode, In-Plane Switching (IPS) Mode, and Vertical Alignment (VA) Mode are known. In recent years, the Optically Compensated Birefringence (OCB) Mode which enables high speed switching is well known. Optical modulation properties vary depending on these liquid crystal modes. For example, viewing angle transmittance, which is an important factor when a LCD is viewed from various angles, usually decreases as a viewing axis departs from the center of a LCD in a direction toward either end of the LCD, that is, as the viewing angle increases. Herein, “viewing angle” means the angle between a viewing axis and a normal line of a liquid crystal panel, whereas “viewing angle transmittance” means the transmittance of a liquid crystal panel obtained at a viewing angle.
In addition, the viewing angle brightness of a backlight drops as the viewer's eyes are directed to a peripheral edge of the screen. Accordingly, when the viewer's eyes move off the center of the screen to an edge of the screen, the brightness seems to gradually decrease to the viewer. Herein, “viewing angle brightness” means the brightness obtained at a certain viewing axis angle with respect to a normal line of the main faces of the optical waveguide of a backlight.
With the OCB mode among the above liquid crystal modes, the viewing angle transmittance varies such that it becomes greater at the edges of the screen than at the center of the screen. In this case, there is the problem that when the viewer's eyes move from the center toward an edge of the screen of the LCD, the brightness increases with its peak in a certain region so that images appear awkward.